Because reading is such a big portion of the test, one of the best ways to prepare is to read challenging books. Many libraries offer reading lists for high school students; choose several books and borrow them for free. Some libraries also have ACT review books in the reference section. You can't borrow these books, but you can read about each section and take sample tests at the library. Be sure to take at least one sample test in a timed condition.
Sign up for Test Prep Review's Free ACT Question of the Day. Every day, you get an e-mail with one ACT question. The website also offers practice questions with an answer key for each ACT section. The official ACT website also offers a free question of the day every day.
Spark Notes Test Prep offers a free online review for each section, along with test taking strategies specific to the ACT. The site offers detailed question analysis for the English, math, reading and science questions. It also offers detailed answers to each question. Even if you get a question right, you might have just guessed correctly; reading the answer can help you better understand the material.